1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, and a facsimile machine, and an optical writing device installed in the image forming apparatus to write images on an image carrier with an optical beam.
2. Discussion of the Background Art
A tandem color image forming apparatus is widely used. In the tandem color image forming apparatus, images (visible images), each having a different color, are formed on a plurality of latent image carriers so as to form full-color images by overlapping these images. The tandem color image forming apparatus includes an optical writing device mounted thereon for forming the visible images. The optical writing device forms latent images on each latent image carrier by irradiating the latent image carrier with an optical beam corresponding to image information and scanning the optical beam. Then, the optical writing device obtains the visible images by developing the latent image formed on each latent image carrier.
The above-mentioned optical writing device generally includes optical elements, such as a lens and a mirror, for directing light emitted from a light source to a predetermined position. The optical elements may include a polygon mirror for deflecting and scanning with the light from the light source to serve as principal scanning-line deflection means. In addition, the optical elements may include a plurality of lenses for focusing the light beam deflected by the polygon mirror on the surface of a latent image carrier which is a light-irradiation target.
In such an optical scanning device, during fabrication certain tolerances may appear in image plane curvature characteristics and positional accuracies of the optical elements. In addition, because a motor that drives the polygon mirror generates heat, various constituent elements of the optical writing device may also be thermally deformed. Moreover, due to changes in weight balance and circumstance of the image forming apparatus, torsion of the image forming apparatus and the optical writing device or torsion of the latent image carrier may be produced during mounting. For these reasons there arises a problem of fluctuation in position and angle between the optical elements, so that the scanning position of the light beam on the latent image carrier fluctuates. Also, for the above-mentioned reasons, a problem arises in that the scanning line of the light beam on the surface of the latent image carrier is curved or inclined.
Due to the problems mentioned above, color misalignment is produced in the full-color images formed by overlapping the images formed on each latent image carrier.
In the problems mentioned above, the positions and attitudes of the lens and the mirror largely affect the relative inclination misalignment of the scanning line. Hence, it is generally required to correct the position and attitude of the optical element (the lens or the mirror) provided for each-color image forming station of the color image forming apparatus. On the other hand, in order to suppress the relative misalignment of the scanning line, it is necessary to structure an optical housing with high accuracies for holding a number of optical elements. Such management of such fabrication tolerance complicates production of the optical housing, so that it is difficult to manage and maintain the production line.
Therefore, an optical writing device has been proposed in which part of the optical elements is held to a punched plate other than a housing having the entire optical elements arranged therein. In this optical writing device, a two-member structure is disclosed in that optical elements are held by a pair of holding members.
Similar optical writing devices capable of improving performance by employing a most suitable material for the function to be achieved have been proposed. Furthermore, there is an optical writing device capable of removing the effect of the thermal expansion/contraction of the two members and an optical writing device in which the first holding member is also made of a plate for increasing the rigidity.
However, in these related arts, if the holding member is divided into two members, problems of error accumulation and relative misalignment remain. In one optical writing device, the torsion of the device itself can be desirably corrected. In this case, however, depending on the structure of the optical writing device, unnecessary torsion may be inadvertently produced.